What is Sink Marks?

Sink marks are shallow depressions or dimples that form on the surface of injection molded plastic parts. They occur when thicker sections of a part cool and shrink at different rates than thinner areas, causing the outer surface to pull inward and create a visible indentation.

These surface defects typically appear opposite features like ribs, bosses, and mounting posts where material thickness increases. While sink marks usually don't compromise a part's structural integrity or function, they create aesthetic flaws that reflect light differently from surrounding surfaces, making them highly visible on finished products.

The Physics Behind Sink Mark Formation

Understanding how sink marks develop requires looking at the thermal behavior of molten plastic during the cooling phase.

When molten plastic enters a mold cavity, it contacts the cooler mold walls and immediately begins solidifying from the outside in. The outer skin forms a rigid shell within seconds, but the interior core remains molten for considerably longer-especially in thicker sections. As this internal plastic continues cooling, it undergoes volumetric shrinkage, contracting by 2% to 20% depending on the material type.

This contraction creates internal tensile forces. If the outer skin lacks sufficient rigidity to resist these forces, it gets pulled inward, forming the characteristic depression we call a sink mark. Conversely, if the surface skin is strong enough to resist deformation, the shrinkage manifests as an internal void instead-a hidden bubble that may compromise structural performance even more seriously than a visible sink.

The differential cooling rate is the primary driver. Consider a part with a 3mm nominal wall intersecting with a boss feature that creates a localized 6mm thickness. The thin wall solidifies in perhaps 8-12 seconds, while the thick boss section may require 30-45 seconds to fully harden. During those extra 30+ seconds, the already-solid thin wall experiences pulling forces from the still-contracting thick section, resulting in surface depression.

Material-specific shrinkage rates play a decisive role:

Semi-crystalline polymers (PP, PE, PBT, POM): 1.5% to 3.0% shrinkage-high sink risk

Amorphous polymers (ABS, PC, PMMA): 0.4% to 0.8% shrinkage-moderate sink risk

Glass-filled compounds: 0.2% to 0.6% shrinkage-lowest sink risk

A 2024 industry survey of injection molders found that sink marks account for approximately 18% to 23% of cosmetic defect rejections in high-visibility consumer products, making them one of the top three quality concerns alongside flash and weld lines.

Primary Causes of Sink Marks

Sink marks don't result from a single factor but rather from the interaction of design, material, and process variables.

Design-Related Causes

Non-uniform wall thickness stands as the single most significant design contributor. When a part transitions from a 2mm wall to a 5mm boss without proper compensation, you're essentially guaranteeing a sink mark. The thick section contains 2.5 times more material volume, which translates to proportionally more shrinkage-shrinkage that the thin adjacent wall cannot adequately support.

Rib and boss geometry creates natural thick sections at intersection points. A standard design approach places a 3mm rib perpendicular to a 3mm wall, creating a 6mm thick junction. Even with a 45-degree draft angle, the base thickness often exceeds recommendations, concentrating material in a way that promotes differential cooling.

Sharp transitions between features exacerbate the problem. Moving abruptly from thin to thick sections doesn't allow the material flow to gradually adjust, creating stress concentration points that become preferential sink locations.

Process-Related Causes

Insufficient packing pressure fails to compensate for natural shrinkage. During the packing phase-which occurs after the cavity fills-additional material should be forced into the mold to offset volumetric contraction. Industry standards suggest packing pressure should reach 50% to 70% of injection pressure, typically 8,000 to 15,000 psi (550 to 1,030 bar) for most thermoplastics. Below this threshold, inadequate material remains in thick sections to prevent surface collapse.

Short holding time creates a related problem. The packing pressure must be maintained until the gate freezes-the point where solidified plastic at the gate prevents any further material from entering or exiting the cavity. For typical automotive components, this gate seal time ranges from 3 to 8 seconds depending on gate dimensions and material thermal properties. Releasing pressure prematurely allows material to flow backward out of the cavity, effectively reducing the amount available to compensate for shrinkage.

Melt temperature deviations affect shrinkage magnitude. Processing above the manufacturer's recommended temperature range (typically specified within a ±10°C window) increases the temperature differential between injection and solidification, amplifying total shrinkage. A PC part processed at 320°C instead of 290°C might experience 15% more volumetric contraction.

Mold temperature control proves equally critical. Recommended mold temperatures typically fall between 80°C and 120°C (176°F to 248°F) for most engineering thermoplastics. Excessive mold temperature delays gate sealing, while insufficient temperature causes premature surface skinning that locks in high internal stresses.

Design Guidelines for Sink Mark Prevention

Preventing sink marks through design optimization represents the most cost-effective approach-fixing issues before tooling begins saves exponentially more than correcting problems during production.

Wall Thickness Optimization

Maintain uniform wall thickness throughout the part. This principle supersedes nearly all others. Target a consistent nominal wall, varying only where absolutely necessary for functional requirements. When variation is unavoidable, transition gradually using chamfers or radii rather than sharp steps.

Recommended wall thickness ranges by material:

PC (Polycarbonate): 1.0mm to 3.5mm (0.040" to 0.138")

ABS: 1.2mm to 3.5mm (0.047" to 0.138")

PP (Polypropylene): 0.8mm to 3.8mm (0.031" to 0.150")

PA (Nylon): 0.8mm to 3.0mm (0.031" to 0.118")

PBT: 0.8mm to 3.0mm (0.031" to 0.118")

Thicker walls increase cycle time and material cost while dramatically raising sink risk. A 4mm wall requires roughly twice the cooling time of a 2mm wall, and the shrinkage-related stresses increase exponentially rather than linearly with thickness.

Rib Design Standards

Ribs add structural rigidity without excessive material use, but improper rib design is a leading sink mark generator.

Critical rib specifications:

Thickness: 50% to 60% of the nominal wall (0.5T to 0.6T). This is non-negotiable for sink prevention. A 3mm wall should connect to ribs no thicker than 1.5mm to 1.8mm at the base. For semi-crystalline materials like PP and PA, stay at the lower end (50% to 55%) due to their higher shrinkage rates.

Height: Maximum 3× the nominal wall thickness (3T). Taller ribs create excessively deep features that complicate venting and cooling. If greater stiffness is needed, add multiple shorter ribs rather than increasing individual rib height.

Draft angle: 0.5° to 1.0° per side. Draft facilitates part ejection and prevents rib top thickness from accumulating excessively. Be cautious with draft angles exceeding 1.5°-the rib becomes too thick at the top intersection (promoting sink) while too thin at the bottom (risking incomplete fill).

Fillet radius: 0.25T to 0.40T. The radius where the rib meets the wall must be generous enough to reduce stress concentrations but not so large that it creates a thick material accumulation. A 0.25T radius typically provides optimal balance.

Base relief: 7° chamfer or radius. Adding a gradual slope at the rib base (transitioning from the rib's nominal thickness to the attachment point) helps material pack more uniformly and reduces the apparent thickness at the junction.

Spacing considerations: Position ribs at least 2T apart (measured between rib faces). Closer spacing restricts cooling channel placement in the mold and can create localized hot spots.

Boss Design Standards

Bosses-cylindrical features designed to accept screws, inserts, or pins-concentrate material in a small footprint and rank among the highest sink mark risk features.

Critical boss specifications:

Outer wall thickness: 60% of nominal wall (0.6T). Never exceed this ratio for amorphous materials; reduce to 50% to 55% for semi-crystalline polymers. A 2.5mm nominal wall should connect to a boss with maximum 1.5mm wall thickness.

Inner diameter: Match the fastener requirements but maximize the hole diameter within functional constraints. Larger holes reduce the boss's effective wall thickness, lowering sink risk.

Boss height: Maximum 2.5× the boss outer diameter. Taller bosses require support ribs to prevent warping during ejection and to improve structural integrity under load.

Base fillet: 0.25T radius minimum. This transition reduces stress concentration and improves material flow around the boss during filling.

Draft angle: 0.5° to 1.0° on both inner and outer diameters. This aids ejection while preventing excessive wall thickness accumulation.

Core-out strategy: For bosses exceeding recommended dimensions, core out the wall immediately adjacent to the boss exterior. This creates a thin-wall tunnel around the boss base, eliminating the thick section that would otherwise cause sinking. The boss then connects to the nominal wall via ribs (following rib design standards above) rather than through continuous thick material.

Strategic Feature Placement

Position bosses near gates whenever possible. Material reaching the boss while still at elevated temperature packs more effectively and experiences reduced pressure drop. A boss located at the far end of a long flow path may not receive adequate packing pressure regardless of process settings.

Avoid placing bosses directly against outer walls. This creates an unavoidable thick section at the intersection. Instead, position the boss slightly inward (minimum 1.5T clearance) and connect it to the wall using properly designed ribs.

Design for material flow direction. Where practical, orient ribs parallel to the anticipated material flow pattern. This minimizes flow resistance and reduces the likelihood of weld lines forming on cosmetic surfaces.

Process Parameter Optimization

Even well-designed parts require proper molding parameters to fully eliminate sink marks.

Packing Phase Control

The packing (hold) phase compensates for material shrinkage by forcing additional plastic into the cavity after filling completes. Think of it as "topping off" the part as it contracts.

Packing pressure guidelines:

Set packing pressure to 50% to 70% of peak injection pressure

For a part requiring 18,000 psi injection pressure, use 9,000 to 12,600 psi packing pressure

Monitor cavity pressure sensors (if available) to verify pressure reaches thick sections

Increase packing pressure incrementally in 200 to 500 psi steps while monitoring for flash

Packing time determination:

The critical parameter is gate seal time-the point where the gate solidifies enough to prevent backflow. Hold pressure must continue until gate seal occurs.

Gate seal verification method:

Run parts at incrementally increasing hold times (5s, 7s, 9s, 11s, etc.)

Weigh each part precisely

Plot weight versus hold time

Gate seal occurs at the hold time where part weight plateaus (additional hold time produces no weight increase)

Set production hold time to seal time plus 10% to 20% safety margin

For most small to medium parts (under 200g), gate seal occurs between 3 to 10 seconds. Large parts may require 15 to 25 seconds.

Temperature Management

Melt temperature optimization:

Follow resin manufacturer specifications (typically a 20°C to 30°C window)

Process at the lower end of the recommended range to minimize total shrinkage

Verify actual melt temperature with a pyrometer at the nozzle-barrel setpoints often differ from true melt temperature by ±15°C

For PC, typical range is 280°C to 310°C; for PP, 200°C to 250°C; for PA6, 260°C to 290°C

Mold temperature settings:

Material-specific recommendations:

ABS: 50°C to 70°C

PC: 80°C to 110°C

PP: 20°C to 60°C

PA6: 60°C to 100°C

PBT: 60°C to 90°C

Differential cooling strategy: For parts where sink occurs on only one side, consider using different coolant temperatures in the core versus cavity. Cooling the problematic side slightly faster can shift the sink to the opposite (non-cosmetic) surface. This technique requires precise thermal management but proves effective for appearance-critical parts.

Cooling Time Extension

Allowing adequate cooling time ensures dimensional stability before ejection. Premature ejection can cause the surface to deform as internal stresses release.

Cooling time calculation: Approximate cooling time (seconds) = (H² × K) / α

Where:

H = maximum wall thickness (mm)

K = material constant (0.5 to 2.0 depending on plastic type)

α = thermal diffusivity of the material

For practical purposes, most engineering thermoplastics require roughly 1 to 1.5 seconds of cooling time per millimeter of wall thickness.

Mold Design Considerations

Mold construction directly impacts sink mark formation through cooling efficiency and venting adequacy.

Cooling Channel Optimization

Conventional cooling uses drilled straight-line channels positioned parallel to the part surface. For sink-prone features, standard cooling often proves inadequate because thick sections sit far from cooling channels.

Conformal cooling follows the part's contours more closely, bringing cooling directly to problem areas. While traditionally expensive (requiring EDM or soldering of drilled channels), additive manufacturing now enables cost-effective conformal cooling through 3D printed mold inserts. A 2024 study of automotive interior components found conformal cooling reduced sink mark depth by 40% to 60% compared to conventional channels in thick-boss areas.

Baffle and bubbler cooling brings coolant flow to isolated features like deep bosses and cores. A baffle is a blade inserted into a core pin, creating a U-shaped flow path. A bubbler uses a tube-within-a-tube arrangement where coolant flows down the inner tube and returns up the outer channel. Both approaches dramatically improve heat extraction from features that would otherwise have poor cooling.

Venting Strategy

Trapped air in thick sections can insulate material from the mold wall, slowing cooling and exacerbating differential cooling rates.

Vent placement: Position vents at:

The last locations to fill (identified via mold flow analysis)

Deep pockets and ribs where air can become trapped

Near thick features like bosses

Vent dimensions:

Depth: 0.01mm to 0.03mm (0.0004" to 0.0012") for most thermoplastics

Width: 5mm to 15mm (0.2" to 0.6")

Deeper vents risk flash; shallower vents restrict air escape

Gate Location

Gate position influences material packing effectiveness. A gate located far from thick sections means material must travel a long flow path, losing temperature and pressure along the way. By the time it reaches the thick feature, it may lack sufficient pressure to pack adequately.

Optimal gate placement:

Position gates to minimize flow path to thickest sections

Use multiple gates for large parts to ensure even packing

Consider hot runner systems for maximum packing pressure retention

Material Selection for Sink Resistance

Different plastic families exhibit vastly different shrinkage behaviors and sink susceptibility.

Amorphous vs. Semi-Crystalline Polymers

Amorphous polymers (ABS, PC, PMMA, PS) have random molecular structure that produces relatively uniform, low shrinkage (0.4% to 0.8%). They're inherently more sink-resistant and preferred for appearance-critical parts.

Semi-crystalline polymers (PP, PE, PA, PBT, POM) develop organized crystal structures during cooling, resulting in higher and less predictable shrinkage (1.5% to 3.0%). They're more sink-prone and require more aggressive design compensation.

Filled and Reinforced Grades

Glass fiber reinforcement dramatically reduces shrinkage. A base PP resin might shrink 1.8%, while 30% glass-filled PP shrinks only 0.3% to 0.6%. The glass fibers create a reinforcing skeleton that resists contraction.

Considerations with filled materials:

Anisotropic shrinkage (different rates parallel versus perpendicular to flow)

Higher injection pressures required due to increased viscosity

Increased tool wear

Visible fiber patterns on surface if wall thickness insufficient

Mineral-filled grades (talc, calcium carbonate) provide intermediate shrinkage reduction at lower cost than glass, though with less mechanical property enhancement.

Emerging Bio-Based Options

Bio-based and recycled-content plastics are growing rapidly. According to industry reports, bio-based plastics in injection molding increased by over 20% between 2023 and 2024, driven by sustainability mandates. However, these materials often exhibit less consistent shrinkage behavior than virgin resins due to composition variability. When using recycled content exceeding 30%, conduct thorough process development to understand the specific material's shrinkage characteristics.

Quality Control and Measurement

Detecting and quantifying sink marks requires appropriate measurement tools and acceptance criteria.

Visual Inspection Standards

Most quality specifications define sink mark acceptability based on visibility under defined conditions:

Standard inspection protocol:

Position part 30cm from observer

Use standard factory fluorescent lighting (400 to 500 lux)

Observe perpendicular to surface and at 45° angle

Observation time: 3 to 5 seconds

Classification:

Visible at 30cm: Reject for Class A (cosmetic) surfaces

Visible only at closer inspection (<15cm): Accept for Class B surfaces

Not visible under normal viewing: Accept for all applications

Quantitative Measurement

3D optical profilometry captures surface topology with micrometer precision. Modern systems scan entire part surfaces in seconds, generating detailed depth maps.

Typical sink mark specifications:

Depth < 0.01mm (0.0004"): Generally acceptable for most applications

Depth 0.01mm to 0.05mm: Borderline; depends on surface finish and location

Depth > 0.05mm: Usually visible and objectionable on appearance surfaces

CMM (Coordinate Measuring Machine) measurement provides precise depth readings at specific locations. A probe or optical sensor measures the depression depth relative to the intended surface profile.

Ultrasonic inspection can detect internal voids that may accompany or substitute for surface sinks. This nondestructive method reveals hidden quality issues before they cause field failures.

Working with an Injection Molding Service

Professional injection molding service providers implement systematic quality control to prevent sink marks from reaching production. Leading services employ moldflow analysis during the design phase to predict sink mark locations before cutting steel. They use cavity pressure sensors during production to verify adequate packing reaches all thick sections. Statistical process control (SPC) monitors cycle-to-cycle consistency in hold pressure, time, and temperature-the critical parameters affecting sink formation.

When evaluating potential partners, look for those offering design for manufacturability (DFM) feedback that specifically addresses wall thickness, rib geometry, and boss design. The best injection molding service teams identify sink risks during quotation and propose design modifications before tooling begins, saving both time and money while ensuring first-article success.

Industry-Specific Considerations

Different industries maintain varying tolerances for sink marks based on application requirements.

Automotive Components

Interior visible parts (instrument panels, door panels, consoles) require Class A surfaces with essentially zero visible defects. The automotive sector, which accounts for over 35% of global injection molding demand in 2024, drives significant investment in sink prevention technologies. Under-hood components tolerate minor surface imperfections since aesthetics are secondary to performance and cost.

Consumer Electronics

Device housings demand flawless appearance. A laptop cover or smartphone case with visible sink marks appears cheap and poorly manufactured, damaging brand perception regardless of functional adequacy. The electronics segment pushes molders toward thin-wall designs (0.8mm to 1.2mm) specifically to minimize sink risk on ribs and bosses.

Medical Devices

Functional requirements dominate aesthetic concerns, but sink marks can indicate process instability that might affect dimensional accuracy or material properties. Regulatory validation requires demonstrating process consistency, making sink mark prevention part of broader quality system requirements.

Packaging

Bottles, containers, and closures generally tolerate minor sink marks unless they occur on highly visible label areas. The packaging industry, representing approximately 32% of injection molding applications, often accepts sink depths under 0.03mm as cosmetically acceptable.

Troubleshooting Existing Sink Marks

When sink marks appear on existing production parts, systematic troubleshooting identifies the root cause and effective correction.

Diagnostic Process

Step 1: Verify molding conditions

Confirm hold pressure meets 50% to 70% of injection pressure target

Verify hold time exceeds gate seal time by minimum 15%

Check melt and mold temperatures against resin specifications

Review recent process changes or material lot changes

Step 2: Assess part design

Measure wall thickness at sink location and adjacent areas

Calculate rib/boss thickness ratio relative to nominal wall

Identify if sink occurs near end of fill (suggesting packing pressure doesn't reach that location)

Step 3: Evaluate material

Confirm material type matches specification

Check for moisture content if using hygroscopic resin (PA, PC, PBT)

Review lot consistency for regrind percentage or formulation changes

Corrective Actions by Priority

First attempts (no design/tool changes):

Increase packing pressure in 300 to 500 psi increments

Extend hold time until gate seal verified

Reduce melt temperature to lower end of processing window

Optimize cooling time to ensure complete solidification

Secondary measures (minor tool modification): 5. Improve cooling at sink location (added baffles, modified water flow) 6. Relocate or resize gates to improve packing pressure delivery 7. Add venting to eliminate trapped air

Last resort (design changes): 8. Reduce rib/boss thickness (requires mold modification) 9. Core out thick sections where possible 10. Redesign features to eliminate thickness variations

Frequently Asked Questions

Why do sink marks appear on some parts but not others from the same mold?

Process variation is the likely culprit. Fluctuations in melt temperature (±5°C), injection pressure (±3%), or hold time (even 1-2 seconds) can push marginal areas into or out of the sink zone. Material lot variations in viscosity or shrinkage rate also contribute. If sink marks appear intermittently, focus on process monitoring and material consistency.

Can sink marks be eliminated through post-molding operations?

Not effectively. Once formed, sink marks represent physical depressions that cannot be removed without material addition. Painting or texture coating can mask shallow sinks (< 0.02mm) by disrupting light reflection patterns, but deeper depressions remain visible. Prevention during molding is the only reliable solution.

Do sink marks affect part strength or just appearance?

For most applications, sink marks are purely cosmetic. The part maintains full structural integrity since the material is fully dense-it's simply recessed. However, sink marks can indicate inadequate packing, which sometimes correlates with reduced dimensional accuracy or increased warpage. If sink depth exceeds 0.1mm, investigate whether other quality issues coexist.

Why do sink marks sometimes appear weeks after molding?

This phenomenon, called "delayed sink," occurs when residual internal stresses slowly release over time, causing gradual surface deformation. It's most common in parts molded with insufficient packing pressure or those experiencing elevated temperatures during storage or use. Once the surface depression forms, it stabilizes. Proper packing pressure and adequate cooling time prevent delayed sink.

Sources:

RJG Inc., "How to Prevent Sink Marks in Injection Molding" (October 2024)

Grand View Research, "Injection Molding Market Size & Share Report" (2024)

Keyence Corporation, "Measuring Sink Marks with 3D Optical Profilers" (2024)

Star Rapid, "Sink Marks Design Guidelines" (June 2025)

FirstMold, "Sink Marks Analysis And Solution" (July 2025)

Aprios, "Sink Marks in Injection Molding: Causes and Fixes" (August 2025)